Well as long as the next game doesn't come out too close to the next console cycle, whenever that may be, I say take your time. Vice City was released a year after 3, and SA was 2 years, look how much more we got out of it. I still loved VC, just saying.

They're probably taking this time to build a huge map, capable of flying planes across, and hopefully without too much pop-up this time (good luck with that, I'd wager 4 already maxed out the PS3+360)